The inventive subject matter relates to power distribution systems and methods and, more particularly, to uninterruptible power supply (UPS) systems and methods.
UPS systems are commonly to provide uninterrupted power in critical applications, such as industrial, medical and data processing applications. The UPS systems used in large-scale distributed electronic installations, such as data centers, may be classified as transformer-type and transformerless. A transformer-type UPS typically includes an output inverter that is connected to the critical load by a transformer that provides galvanic isolation between the load and the input to the UPS (e.g., an AC utility source or generator). Transformerless UPS designs omit the output transformer and typically connect the output inverter directly to the load. Potential advantages of transformerless designs include elimination of the bulk, weight and cost associated with the output transformer. An example of a transformerless UPS design is the Eaton 9395 UPS, described in Eaton 9395 UPS 225-1100 kVA (December 2010), available at powerquality.eaton.com.
Transformerless UPS designs may encounter operational problems when used in three-wire installations in which an input source neutral conductor is not provided to the UPS. These problems may include unstable performance, grounding compliance issues and/or system conflict issues in paralleled applications. For a manufacturer of transformerless UPSs, such issues can lead to loss of sales, as the aforementioned problems can lead consultants to refrain from specifying transformerless designs, even though they may offer advantages in cost, weight and space.
One technique to address such issues includes installation of a set of wye-connected inductors at the UPS to provide a neutral reference in three-wire applications. An example of such an approach is illustrated in FIG. 15, which schematically illustrates a neutral reference circuit 5 for use with a transformerless UPS 2, along the lines of a “neutral reference kit” offered by Eaton Corporation for use with its 90 kW 9390 three-phase UPS in three-wire service applications. The neutral reference circuit 5 includes three inductors connected in a star configuration. Respective ones of the inductors are connected to respective phases of a bypass input of the UPS 2, with a central connection point of the inductors being coupled to a neutral point N of the UPS 2. The neutral point N is internally connected to control circuitry 3 of the UPS 2, which uses the neutral point N as an input for monitoring and control operations of the UPS 2.